A spring has a natural length of 26 cm. If a 21 N force is required to keep it stretched to a length of 28 cm, how much workWis required to stretch it from 26 cm to 32 cm?

I used hookes law, F=kx and found k as 525x. So I integrated 525x with the intervals [.028 to .032] it was wrong.

If 8 J of work are needed to stretch a spring from 11 cm to 13 cm and 12 J are needed to stretch it from 13 cm to 15 cm, what is the natural lengthLof the spring?

I do not understand how to do this problem.. I never learned how to do Work in high school so this is very new to me.